Support lazy CSS generation in Input - #2165

Closes #1945

Allow Input to receive a function that generates its CSS on demand:

const input = new Input(() => generateCSS(), { from: 'a.scss' })
input.from // Does not generate the source
input.css // Generates and caches it

The lazy path reuses the existing constructor for BOM handling, documents, and source maps. Successful initialization replaces the accessors with ordinary writable properties. If initialization throws, the input can be retried. String and Buffer inputs keep their existing eager behavior.

Without from, resolving the input identifier may generate the source because a previous source map can supply the filename. The constructor documentation describes this distinction.

Added tests for deferred generation, caching, BOMs, source maps, writable properties, failed initialization, JSON round-tripping, and error positions. The new tests fail against the original implementation.

pnpm test passes on Arch Linux with Node.js 26.10.0 and pnpm 11.28.4: 731 unit tests, 100% line coverage, lint, types, version, integration, and size checks. Other Node versions and Windows have not been run locally.

Summary by CodeRabbit

• New Features
  • Input sources can now be provided as a function that generates CSS. Generation is deferred until the source or its source map is needed, then cached after successful initialization.
  • Inputs with a specified source path use that path immediately, without triggering deferred generation when reading the input’s path or ID.
  • Lazy-generated inputs support source maps, serialization, and writable input properties.